The present invention relates to a focus controlling method and system, and, more specifically, to a focus controlling system for an image capturing system. The magnification ratio of the system can be controlled through a closed loop control method and system according to the variation in the position of the object being scanned.
An image capturing system like a scanning system is an image processing tool which is popularly employed for acquiring the image of documents, photographs, or even objects. The image is acquired and then transferred to an accessible signal or data for a processor or a computer. Without limiting the scope and application of the present invention, the operation of a scanner is illustrated as follows. In general, the document is illuminated by a light source when being scanned. The image is then transferred onto image or photo sensing devices through one or a set of lenses. Generally speaking, the photo sensing devices like CCD (charge-coupled device), CIS (contact image sensor), or photo-sensitive MOS (metal oxide semiconductor) devices, can be employed. The photo sensing devices can transform the image to accessible signals. In general, the scanning systems can generally be classified to at least three categories by its operation characteristics, including a handhold scanning system, a sheet feeding scanning system, and a flatbed scanning system.
For achieving the high quality of the image being captured, a flatbed scanning system is used frequently. FIG. 1 illustrates a schematic side view of a portion of a flatbed scanning system. In the application of the flatbed scanning system, a document or an object being scanned 10 is placed on a transparent plate 12. An image capturing device 14 is moved under the transparent plate 12 to capture the image of the document 10 line-by-line. The quality of the image acquired is greatly dependent on the characteristics of the image capturing device 14. The parameters like resolution and depth of focus (DOF) have a great influence on the quality of the image transferred.
Under the raised requirement of image quality and the increasing processing ability of related processors or computers, the resolution of the scanner is significantly raised to capture high quality images. In general, the depth of focus of the image capturing device 14 is quite limited, especially for high resolution scanning systems. However, in most of the image capturing applications, some documents 10 are covered with a document wrapper or a transparent document wrapper 10a, as shown in FIG. 2. The insertion of the document wrapper 10a between the document 10 and the image capturing device 14 increases the distance between the two. The optical path between the object being scanned and the lens of the optical system is then changed. If the focus point, or namely the length of the optical path, of the image capturing device 14 is remained unchanged, the magnification ratio would be varied under the increased object distance. The change of magnification ratio on the scanned width might cause the captured image to be distorted in size or the length-to-width ratio. Besides, the object being scanned can be left outside the range of designed depth of focus, the resolution and clearness of the image is greatly influenced. The image quality is then damaged under the shifted focus point.
For most image scanning devices, the focus point of the object being scanned is fixed. For high resolution scanning systems, the depth of focus is generally very short and the tolerance to the variation in the document height is extremely low. However, lots of document are scanned accompanied by a document wrapper with a considerable thickness. The object distance of the document 10 from the image capturing device 14 is increased. In order to adapt for the change of document height, an improved scanning system must be provided with an adaptive focus controlling system. Therefore, the magnification ratio and the resolution of the scanning process can be adjusted and the quality of the image captured can be maintained.
The present invention discloses a focus controlling method and system for an image capturing system. The length of the optical path between the scanned object and the lens of the image capturing device can be controlled. Under the variation in the position of the scanned object, the magnification ratio of the system can be controlled or maintained within a tolerable range through a closed loop control method and system.
The focus controlling system of the present invention can include a calibration paper, an image capturing device, an input interface, and a processing and controlling device. The image capturing device is employed for scanning the calibration paper. The image capturing device has a focus point adjusting mechanism. The input interface is utilized for receiving a location data of a scanning point. The processing and controlling device is responsive to the input interface and the image capturing device to control the focus point adjusting mechanism. As an example, the image capturing system can be a flatbed scanning system.
The focus controlling method of the present invention includes the following steps. First, a scanning point is set or detected and a compensated magnification ratio for the scanning point is then calculated. A focus point of a image capturing device is adjusted and a calibration paper is scanned. The adjusting step and the scanning step are then repeated until a magnification ratio of the calibration paper is approximately equal to the compensated magnification ratio. The focus controlling process is then finished with a justified magnification ratio for the raised height of the object being scanned.